A solar cell is any device that directly converts the energy in light (i.e., light energy or photons) into electrical energy through the process of photovoltaics, which is also referred to as the “photovoltaic effect.” A photovoltaic module is a packaged, connected assembly of solar cells. A solar panel is a set of solar photovoltaic modules electrically connected and mounted on a supporting structure. The majority of solar modules use wafer-based crystalline silicon cells or thin-film cells based on cadmium telluride or silicon. Most solar modules are rigid, but semi-flexible ones are available, based on thin-film cells.
Currently, most commercial solar cells are made from a refined, highly purified silicon crystal, similar to the material used in the manufacture of integrated circuits and computer chips (wafer silicon). The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies such as polymer solar cells.
A polymer solar cell is a type of flexible solar cell made with polymer chains formed from large molecules with repeating structural units that produce electricity from sunlight by the photovoltaic effect. Polymer solar cells include organic solar cells (also called “plastic solar cells”). They are one type of thin film solar cell; others include the currently more stable amorphous silicon solar cell. Polymer solar cell technology is relatively new and is currently being very actively researched.
Compared to silicon-based devices, polymer solar cells are lightweight, potentially disposable and inexpensive to fabricate, flexible, customizable and they have lower potential for negative environmental impact. The major disadvantage of polymer solar cells is that they offer about ⅓ of the efficiency of hard materials. They are also relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.
Organic polymer solar cells (“OPSCs”) differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction (i.e., the boundary or interface between two types of semiconductor material) to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices.
The performance of any type of solar battery is closely dependent on the conductivity of the electrode material and the adhesion of the electrode to the solar cell film. It has recently been proposed to use graphene and graphene oxide for the cathode and anode respectively since the two materials have appropriate work functions for the hole and the electron, respectively, and are resistant to oxidation. Adhesion of the graphene materials to the polymer films, though, still poses a problem. The polymer surface of the OPSC is hydrophobic and, hence, not amenable to any water-lift off technique for processing or plating. Spin coating is problematic since hydrophobic organic solvents sometimes used to disperse the graphene also dissolve the underlying polymer film and disturb the surface properties. Chemical vapor deposition of graphene at 1000° C. also degrades the underlying active polymer layer. Therefore, there is a need for a method for easily and economically depositing graphene on the surface of a polymer layer. There is also a need for a polymer solar cell that can generate electricity more efficiently than the polymer solar cells now being used.